That doesn't match my experience, or most of the stuff I've read. The number of calories I need to hit to lose weight has been pretty constant from my early twenties to late thirties.
That said, I didn't have to be as careful in my twenties because I did a lot more exercise. And that's because I had more free time and opportunity for sports, fewer energy demands, less money for food, and more incentive to walk or cycle places. So I agree it's probably easier for university students to be slim, but I suspect metabolism is not a primary reason.
20s to 40s metabolism isn't that different. 4-5 pints is nearly 1000 calories, and basically nobody has a 1000 cal decline in base expenditure from 20 to 40.
A bigger change, generally, from someone in college and someone in their 40s is their activity levels. Even just considering the amount of walking most folks do on a college campus is a huge difference, compared to someone that gets in their car and drives to and from work.
That said, I didn't have to be as careful in my twenties because I did a lot more exercise. And that's because I had more free time and opportunity for sports, fewer energy demands, less money for food, and more incentive to walk or cycle places. So I agree it's probably easier for university students to be slim, but I suspect metabolism is not a primary reason.